1. Field of the Invention
Embodiments of the invention relate to a semiconductor device and a method of manufacturing a semiconductor device.
2. Description of the Related Art
Conventionally, silicon (Si) is used as a constituent material of power semiconductor devices that control high voltage and/or large current. There are several types of power semiconductor devices such as bipolar transistors, insulated-gate bipolar transistors (IGBTs), and MOSFETs. These devices are selectively used according to an intended purpose.
For example, bipolar transistors and IGBTs have high current density compared to MOSFETs, and can be adapted for large current but cannot be switched at high speeds. In particular, the limit of switching frequency is about several kHz for bipolar transistors and about several tens of kHz for IGBTs. On the other hand, power MOSFETs have low current density compared to bipolar transistors and IGBTs, and are difficult to be adapted for large current but can be switched at high speeds up to about several MHz.
Nonetheless, there has been a strong demand in the market for a power semiconductor device achieving both large current and high speed. Thus, IGBTs and power MOSFETs have been intensively developed and improved, and the performance of power devices has substantially reached the theoretical limit determined by the material. In terms of power semiconductor devices, semiconductor materials replacing silicon have been investigated and silicon carbide (SiC) has been focused on as a semiconductor material enabling production (manufacture) of a next-generation power semiconductor device with a low ON voltage, high-speed characteristics, and high-temperature characteristics.
Silicon carbide is chemically a very stable semiconductor material, has a wide bandgap of 3 eV, and can be used very stably as a semiconductor even at high temperatures. Silicon carbide has a critical electric field strength that is ten times that of silicon or greater, and thus is expected to be a semiconductor material that can sufficiently reduce ON resistance. These merits of silicon carbide are common to other semiconductors (hereinafter, wide bandgap semiconductor) having a bandgap greater than that of silicon, such as gallium nitride (GaN). Thus, lower resistance and higher voltages of a semiconductor device can be achieved by using a wide bandgap semiconductor.
The efficiency of a power semiconductor device using a wide bandgap semiconductor may be improved by reducing the ON resistance. When a trench-type MOSFET is used for a conventional planar MOSFET, a shorter cell pitch and higher mobility may be obtained, enabling reduction of the ON resistance (for example, refer to Tsunenobu Kimoto and James A. Cooper, “Fundamentals of Silicon Carbide Technology”, pp. 320-324, IEEE Press, 2014).
FIG. 18 is a cross-sectional view of a configuration of an active region of a trench-type MOSFET of a related art. The trench-type MOSFET has, for example, an n−-type silicon carbide epitaxial layer 1 on an n+-type silicon carbide substrate 2, plural p+-type bases layer 3 formed in an n-type current spreading layer (hereinafter referred to as CSL layer) region 15 on the n−-type silicon carbide epitaxial layer 1, a p-type channel region 16 and an n+-type source region 17 on the n-type CSL layer region 15, a p+-type region 18 in the p-type channel region 16 and the n+-type source region 17 and in contact with a p+-type base layer 3, a trench 19 formed from a front side toward a p+-type base layer 3, a gate electrode 20 of poly-silicon embedded in the trench 19, a field insulating film region 21 formed on the trench 19, and a source electrode region 22 formed on the front side of the substrate.